This present invention relates generally to semiconductor manufacturing equipment. More specifically, the present invention relates to controlling the position of a semiconductor substrate during processing. Merely by way of example, the invention has been applied to centering a semiconductor wafer inside a processing chamber. The method and apparatus can be applied to other applications as well such as positioning of disk drive substrates, flat panel display substrates, mechanical substrates, and the like.
The processing of semiconductor wafers to form integrated circuits and the like generally requires a number of sequential processing steps. Generally, these processes include steps to create devices, conductors, and insulators on the substrate. Often, these processes are performed in a number of processing chambers, each chamber dedicated to a single process. Some semiconductor processing systems utilize a central transfer chamber to couple these dedicated processing chambers, forming a “cluster tool.” Examples of these cluster tools include the families of PRODUCER®, CENTURA®, AND ENDURA® processing systems available from Applied Materials, Inc., of Santa Clara, Calif.
Generally, a cluster tool includes a central transfer chamber that houses a robot to facilitate transfer of the substrate between the surrounding processing chambers. This central transfer chamber is generally coupled to at least one load lock chamber and one or more processing chambers. In some cluster tools, multiple robots are located in the central transfer chamber to facilitate the transfer of semiconductor wafers from the load lock chambers to the processing chambers. Generally, semiconductor wafers are stored in wafer cassettes and transferred to the central transfer chamber via the load locks in preparation for processing. The processing chambers are typically utilized to perform various processing steps such as etching, physical vapor deposition, chemical vapor deposition, ion implantation and the like. During wafer transfer operations, semiconductor wafers are supported on moveable wafer transfer blades.
In order to accurately place the semiconductor wafers in a given processing chamber, control over the wafer handling and transfer process is typically exercised by the cluster tool. As the size of semiconductor device features has decreased, processing tolerances have become more stringent and the accuracy requirements for wafer handling have increased. For example, in some processing steps, the placement tolerance for positioning the wafer in the processing chamber has been reduced to smaller dimensions.
Accurate placement of the semiconductor wafers in the processing chambers may be hindered by a variety of factors. For example, thermal expansion of the components that make up the wafer transfer blades may shift the position of the semiconductor wafer from a desired position. Additionally, motion of the wafer transfer blades may result in the semiconductor wafer sliding on the wafer transfer blade during motion of the wafer transfer blade. Consequently, the placement position of the semiconductor wafer inside the processing chamber may be inaccurate.
Several methods and apparatus have been utilized to determine the position of wafer transfer blades and other components of a robotic transfer system. In one approach, the position of a wafer is determined by measuring a reference position of a robot utilized to transfer a wafer between chambers of a semiconductor manufacturing system. However, although the position of a robot and an associated wafer transfer blade may be accurately determined, the position of a semiconductor wafer supported by such a wafer transfer blade may be unknown. For example, although a semiconductor wafer may be accurately positioned on a wafer transfer blade at the location where the wafer is removed from the wafer cassette, the wafer may slide during the movement of the robotic assembly through the load lock and transfer chamber. In fact, sliding of the wafer on the wafer transfer blade may cause the wafer to become misaligned in an undetermined manner. Therefore, in the general case, even though the position of the robot or the wafer transfer blade may be accurately determined and controlled, the position of the semiconductor wafer may be undetermined.
In another approach, an optical emitter that produces a collimated beam of light is disposed on the surface of a transparent cover on the surface of the wafer transfer chamber. Detectors also mounted on the surface of the chamber are tripped when the beam of light is interrupted by either the wafer or the wafer transfer blade. Thus, the system can determine the position of the wafer with respect to the wafer transfer blade. Based on the detected position, the system can correct for wafer position errors. Sensor systems of the type known to be used in this approach are generally accurate to ˜5 mils. As semiconductor device dimensions have decreased, increased handling accuracy is desirable, thus providing motivation for wafer handling apparatus with increased accuracy.
Therefore, there is a need in the art for an improved method and apparatus for positioning and continuously monitoring the position of a semiconductor wafer in a processing chamber.